stop using dict iter* helpers

Change from iter{keys,items,values}() to {keys,items,values}().
Python 3 doesn't offer the iter variants, and we don't really have
any cases where performance is critical where we'd need to avoid
creating the temporary list (which is what Python 2 does).

This is mostly mechanical.

BUG=chromium:980619
TEST=lint is unchanged in chromite
TEST=unittests pass

Change-Id: I591110f2eb5d43bb9268b975b14bd7ade66a4e35
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1689014
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
diff --git a/scripts/sysmon/system_metrics.py b/scripts/sysmon/system_metrics.py
index 359ab82..ae6d057 100644
--- a/scripts/sysmon/system_metrics.py
+++ b/scripts/sysmon/system_metrics.py
@@ -152,7 +152,7 @@
 
 def _collect_disk_io_info():
   try:
-    disk_counters = psutil.disk_io_counters(perdisk=True).iteritems()
+    disk_counters = psutil.disk_io_counters(perdisk=True).items()
   except RuntimeError as ex:
     if "couldn't find any physical disk" in str(ex):
       # Disk performance counters aren't enabled on Windows.